1. Field of the Invention
This invention relates to the field of color image forming apparatuses such as digital copying machines or color facsimile machines, and in particular relates to an apparatus for controlling an under color removal process used for generating a black component using color image signals of an original document.
2. Description of the Related Art
A color image forming apparatus such as a digital copying machine or a color facsimile machine which optically reads an original image of a document and obtains color image signals with an image scanner, processes the signals, and prints the color image is well known. In this kind of apparatus, it is important to perform appropriate signal processing on the image signals with respect to characteristics of the image, depending on whether the input image is a natural color photographic image or a monochrome text image. When a natural color photographic image is formed by the apparatus, basic colorants such as cyan (C), magenta (M), yellow (Y) and black (Bk) are used to print the image. When the original document includes only monochrome text, the image forming operation may only use the black colorant (Bk).
Some types of image forming apparatuses provide a discrimination of the chromatic range of an inputted document image which is used for controlling the image forming operation. The discrimination determines whether the inputted image includes chromatic components in a predetermined amount or more. Based on the discrimination, the apparatus controls whether the image forming operation therein is a full color operation or a black and white operation. The time consumed during the image data processing and forming of full color images is much longer than the time required for forming monochrome images. According to this discrimination, the apparatus might detect the existence of a binary text region or a natural color photograph region in the inputted image. For example, this kind of apparatus is disclosed in U.S. Pat. No. 5,208,663, which is incorporated herein by reference.
For detecting input image characteristics, there are two kinds of conventional systems used in image forming apparatuses. The first one is a pre-scan system which uses an additional scanning operation for discriminating input image characteristics before the image forming operation starts, and determines the condition and parameters which will be used during the actual image forming operation. A subsequent scanning operation for actually forming the image using the previously determined conditions is then performed.
The second type of conventional system is a pre-scanless system which performs an initial image forming operation during a first scanning operation. Typically, the initial image forming operation is an operation using the black colorant. The pre-scanless system detects color characteristics of the input image such as the amount of the chromatic components in the image which is obtained in the first scanning operation. If a predetermined chromatic component is detected in the first scanning operation, the image forming apparatus detects that the input image is a color image. Accordingly, the apparatus is changed from initial image forming operation (e.g. black component) to the chromatic image forming operation during the second scanning operation for image forming.
In the image forming apparatus using the pre-scan system, the copying time of the document is increased because of the additional scanning operation necessary for image discrimination. Therefore, productivity of document duplication and work efficiency is reduced. However, even in the pre-scanless system, there is a reduction in the quality of the image or image fidelity. The pre-scanless system initially performs the black colorant operation of the image forming operation. When the image forming operation is for a color image, the pre-scanless system is limited in the amount of under color removal (UCR) which can be performed for each of color image signals.
In a pre-scanless system, the formed image might be limited in the amount of the black components which can be eliminated from each of input color signals so as to avoid generation of noise or darkness in a highlight region of the color image. However, if the image is determined to be an achromatic image (e.g. a black and white image), the recording density of the Bk colorant in the formed image would be reduced and image fidelity reduced because other colorants used for forming chromatic images are not used. Therefore, the formed image would lack sufficient recording density and resolution. Further, when the scanned image is a color image but the first scan is used to generate the black component, a recording density of the Bk component would be so dark in the formed image that the formed image would lack quality and become noisy, especially in the highlight region of the chromatic image, even though the black component would be appropriate for a black and white document.